THIS INVENTION relates to slurries. More particularly, the invention relates to a method and to an installation for forming and maintaining, for consumption, a stock of slurry.
According to one aspect of the invention there is provided a method of forming and maintaining for consumption a stock of slurry derived from a liquid and from a particulate solid material, the method including the steps of:
withdrawing a slurry of the liquid and the solid material from a stock of the slurry in a reservoir;
directing a stream of the liquid or of the withdrawn slurry at a bulk supply of the solid material in the form of a heap of the solid material, and on to the solid material of the heap;
allowing the stream directed at the heap to drain under gravity from the heap into the reservoir, the stream, while draining from the heap, entraining therein solid material from the heap to increase the specific gravity of the stream and to carry solid material from the heap into the stock of slurry in the reservoir to increase the specific gravity thereof; and
feeding liquid to the reservoir to maintain the volume of the slurry in the reservoir, the method including controlling both the volume of slurry in the reservoir and the specific gravity of the slurry in the reservoir, the feeding of the liquid into the reservoir being initiated, or having its feed rate increased, in response to decreases in the volume of the slurry in the reservoir associated with withdrawal of slurry from the reservoir for consumption thereof, and the directing of the stream at the heap of solid material being initiated, or having its flow rate increased, in response to decreases in the specific gravity of the slurry in the reservoir associated with feeding the liquid to the reservoir, thereby both to maintain the volume of slurry in the reservoir at a desired value, and to maintain the specific gravity of the slurry in the reservoir at a desired value, the feeding of liquid to the reservoir being reduced in rate or discontinued when the desired slurry volume is regained, and the directing of the stream at the heap being reduced in rate or discontinued when the desired specific gravity value is regained.
It will be appreciated that increases and decreases to the specific gravity of the slurry are associated respectively with increases and decreases to the solids content thereof.
While it is in principle possible to carry out the method by directing a stream of the liquid at and on to the solid material of the heap, to form a slurry having a solids content and specific gravity greater than the solids content and specific gravity respectively of the slurry in the reservoir, to permit drainage of a slurry from the heap into the reservoir which increases the solids content and specific gravity of the slurry in the reservoir, it is preferable to employ slurry withdrawn from the reservoir for this purpose. The stream directed at the heap may thus be a stream of the slurry withdrawn from the reservoir.
The method of the invention contemplates that withdrawal of slurry from the reservoir for consumption thereof may be continuous or intermittent, and that the liquid feed to the reservoir, which may be continuous or intermittent, will be at a rate such that the volume of slurry in the reservoir is maintained at the desired value when slurry is withdrawn for consumption at any rate up to the maximum intended rate. Similarly, the withdrawal of the stream of slurry from the reservoir to direct at the heap, which withdrawal may also be continuous or intermittent, will in turn be at a rate such that the specific gravity of the slurry in the reservoir is maintained at the desired value when the slurry is withdrawn for consumption at any rate up to the maximum intended withdrawal rate for consumption. Thus, the desired volume of the slurry, at least on average and with no more than acceptable departures from the desired value, can be maintained in the reservoir while the specific gravity, ie the density or solids content, of the slurry is likewise maintained, at least on average and with no more than acceptable departures from the desired value. In a particular embodiment of the invention, when the stream directed at the heap is a stream of slurry withdrawn from the reservoir, the liquid feed to the reservoir may be intermittent, being at a fixed rate no less than the maximum rate at which the slurry is withdrawn from the reservoir for consumption, the stream directed at the heap being withdrawn from the reservoir intermittently and at a fixed rate.
In a further particular embodiment of the invention the liquid may be water, the solid material being particulate or powdered limestone, or possibly dolomite. In other words, the liquid may be water, the solid material being selected from the group consisting of limestone, dolomite and mixtures thereof.
The controlling of the volume of slurry in the reservoir may be automatic, being by controlling the depth of slurry in the reservoir by means of a valve, the valve being selected from the group consisting of ultra-violet (UV)-controlled valves and, preferably, float-controlled valves and the valve controlling the liquid feed to the reservoir. Similarly, controlling the specific gravity of the slurry in the reservoir may be automatic, for example by using one or more load cells on which the reservoir is supported, to measure the mass of the reservoir, or by using one or more pressure cells in the interior of the reservoir to measure the static pressure in the reservoir, said cell or cells controlling the withdrawal of the stream of liquid from the reservoir which is directed at the heap. In other words, the controlling of the specific gravity of the slurry in the reservoir may be automatic, being by using a cell selected from load cells which measure the mass of the reservoir and pressure cells which measure static pressure in the reservoir, to control the directing of the stream at the heap. The liquid feed to the reservoir or the slurry stream withdrawal from the reservoir may be provided with on/off flow control, and/or they may incorporate variation of their flow rates in response to the magnitude of any departure of the slurry volume or specific gravity from the respective desired values thereof.
Indeed, instead of using load cells or pressure cells, the reservoir can in principle float in a body of water, the depth at which the reservoir floats in the body of water being related to the mass of the reservoir and hence to the specific gravity of the slurry therein, changes in this depth being used to control the directing of the stream at the heap, although it is expected that use of load cells or pressure cells will be preferred.
The method may involve agitation of the slurry in the reservoir to promote maintenance of the solid material in suspension in the liquid and to resist its settling out from the liquid, and the method may include replenishing the bulk supply of particulate material in the heap, as and when required, e.g. intermittently by means of vehicles loaded therewith Withdrawal of slurry from the reservoir, either as a stream to be directed at the heap, or for consumption thereof, may be by pumping it, and directing the stream of slurry at the heap may be by spraying it, e.g. from one or more spray nozzles at the ends of respective hoses, which may be hand-held. In particular, the method may include the step of agitating the slurry to promote maintenance of the solid in suspension in the liquid, withdrawal of slurry from the reservoir being by pumping, directing the stream at the heap being by spraying it from one or more hand-held nozzles, and feeding the liquid to the reservoir being by directing it at the heap.
If desired, the method may make provision for the removal of stones and/or grit from the slurry, between the reservoir and the hose or hoses, e.g. by passing the slurry upwardly through an inverted-conical tank, the slurry from the reservoir being fed into the tank at a low level where its diameter is small, and the slurry issuing from the tank at a high level where its diameter is large, the tank shape and size and the slurry flow rate being selected to promote retention of stones and grit in the tank, while an overflow from the tank takes the place of slurry containing particles of acceptably small size, and stones and grit are removed, e.g. intermittently from the lower parts of the tank; and slurry can, if desired, be recirculated from the top of the tank to its bottom, to achieve upward flow rates in the tank which keep small slurry particles in suspension.
The feeding of the liquid to the reservoir to maintain the volume of slurry in the reservoir may be directly from a liquid supply into the reservoir, or it may be indirect, the liquid being fed from the liquid supply into the stream of slurry which is directed at the bulk supply, so that the liquid enters the reservoir as part of the slurry draining into the reservoir
According to another aspect of the invention there is provided an installation for forming and maintaining, for consumption, a stock of slurry derived from a liquid and from a bulk supply of particulate solid material, the installation including;
a support having an upwardly facing support surface for supporting a heap of particulate solid material, the support surface sloping so that it is inclined to the horizontal to permit a slurry of liquid and particulate solid material to drain under gravity therefrom;
a reservoir arranged to receive slurry draining under gravity from the support surface of the support and to hold a stock of slurry;
a liquid feed for feeding liquid to the reservoir;
a slurry withdrawal device for withdrawing slurry from a stock of slurry in the reservoir;
a spray device for spraying a liquid or slurry at a heap of particulate material supported on the support and on to the particulate material of the heap;
a volume control device for controlling the volume of the stock of slurry in the reservoir and operative, in response to a decrease in said volume below a desired value, to initiate, or increase the rate of, the feeding of liquid by the liquid feed to the reservoir; and
a specific gravity control device for controlling the specific gravity of the stock of slurry in the reservoir and operative, in response to a decrease in said specific gravity below a desired value, to initiate, or increase the rate of, the spraying of the liquid or slurry at the heap,
the volume control device being operative to discontinue, or to reduce the rate of feeding of, liquid to the reservoir, when the desired volume is regained and the specific gravity control device being operative to discontinue, or to reduce the rate of spraying of, the liquid or slurry at the heap, when the desired specific gravity is regained.
The support may be a cementitious, e.g. concrete, slab resting on the ground and having a shaped support surface for supporting the heap, the support surface having, for example, two flat portions which slope and converge downwardly to meet at a line of intersection therebetween, which line in turn slopes downwardly towards a drainage point where the support surface drains into the reservoir. The slab may have a pair of ramps, respectively sloping upwardly from ground level to peripheral edges of the flat portions of the support surface, to facilitate the tipping of particulate material from load vehicles on to the support surface. In a particular embodiment of the installation, the support may thus be a slab of cementitious material having a shaped support surface for supporting the heap, the support surface sloping downwardly to a drainage point and the slab being provided with a vehicle ramp leading upwardly from ground level to a peripheral edge of the slab, which edge is raised above ground level, the reservoir being a completely mixed tank located below the level of the drainage point and provided with a stirrer in its interior.
The reservoir may in turn be in the form of an open-topped completely mixed tank, located in a pit below ground level, at a position where the support surface can drain under gravity into the open top of the tank, and having a stirrer in its interior.
The slurry withdrawal device may be a pump. The pump for withdrawal of slurry for consumption may simply feed into a pipe or similar flow line; and there may be a further similar pump for withdrawal of a stream of slurry to be directed at the heap, which further pump may feed Into one or more hoses, each optionally provided with a spray nozzle at its free end. Thus, the spray device may comprise one or more hoses, each of which has a free end provided with a spray nozzle.
The volume control device may be in the form of a liquid level control means and may thus be an ultra-violet (UV)-controlled valve or preferably a float-controlled valve controlling feeding of liquid from the liquid feed, which may be a pipe, into the reservoir, the valve conveniently being a shut-off or on-off valve arranged to open fully and permit feeding of liquid at a fixed rate into the reservoir if the level of slurry in the reservoir decreases below a desired value, equivalent to the desired slurry volume in the tank, and to shut off liquid flow from the pipe when said desired level is regained. Accordingly, the volume control device may be a shut-off valve selected from the group consisting of ultra-violet (UV)-controlled valves and float-controlled valves, the shut-off valve being arranged to open if the level of slurry in the reservoir decreases below a desired value and to close when said desired value is regained.
The specific gravity control device may be one or more load cells on which the tank is supported in the pit, arranged to control withdrawal of the stream of slurry from the tank by automatically starting the associated pump, or increasing its pumping rate, when the mass of the tank and the desired volume of slurry in the tank contents decreases below a predetermined value corresponding to the desired specific gravity, and arranged automatically to discontinue the pumping, or reduce its rate, when the predetermined value is regained Thus, generally, the specific gravity control device may comprise at least one cell selected from the group consisting of load cells on which the reservoir is supported for sensing the mass of the reservoir, and pressure cells in the interior of the reservoir for sensing static pressure in the reservoir, the cell being operatively connected to a shut-off valve for supplying liquid or slurry to the spray device.
In this regard it will be appreciated that spraying slurry on to the heap and allowing it to drain under gravity into the tank while entraining particulate solid material from the heap, will increase the concentration of solid material in the slurry in the tank and hence will increase the mass of the slurry and hence will increase the total mass of the tank and its contents when filled with the desired volume of slurry. Correspondingly, feeding liquid into the tank will dilute the slurry and reduce the mass of the slurry and hence reduce the total mass of the tank and its contents when filled with slurry to the desired volume or level. Withdrawal of slurry from the tank for consumption leads to feeding liquid into the tank and to a decrease in specific gravity of the slurry, which in turn leads to spraying of slurry on to the heap and washing solid material from the heap into the tank, thereby causing a compensatory increase in specific gravity. Thus, both the volume or level of slurry in the tank, and its specific gravity, can automatically be maintained.
Instead of comprising one or more load cells, the specific gravity control device may comprise one or more pressure cells in the tank, e.g. at or near its bottom, arranged to control the withdrawal of the stream of slurry from the tank by automatically starting the associated pump, or increasing its pumping rate, when the pressure sensed by the pressure cells decreases below a predetermined value corresponding to the desired specific gravity, the pressure cells being arranged automatically to discontinue the pumping, or to reduce its rate, when the sensed pressure regains its predetermined value. In this regard it will be appreciated that the pressure exerted by the slurry, at a particular depth, is directly related to the specific gravity of the slurry which in turn is, directly related to the solids concentration in the slurry. The pressure cell or cells are thus functionally equivalent to the load cell or cells, the cell or cells in question in each case being, for example, electrically connected to a switch or pump flow control means for switching the associated pump on or off or for controlling its pumping rate, which pump is thus conveniently electrically driven. In particular, the specific gravity control device may comprise at least one cell selected from the group consisting of load cells on which the reservoir is supported for sensing the mass of the reservoir, and pressure cells in the interior of the reservoir for sensing static pressure in the reservoir, the cell being operatively connected to a shut-off valve for supplying liquid or slurry to the spray device.
The pump for withdrawing the slurry stream from the tank, and the pump for withdrawing slurry for consumption from the tank, may each be submersible pumps suspended in slurry in the tank, or, conveniently, may each be outside the tank having inlet pipes projecting downwardly into the slurry in the tank, in each case extending downwardly via the open top of the tank into the slurry, so that the mass of or pressure in the tank is essentially unaffected thereby, there being no direct physical connection between said pumps (or the their inlet pipes), and the tank walls of floor, particularly when load cells are employed. If desired, the inlet pipes to the pumps which project downwardly from the pipes into the slurry, may be replaced by flexible pipes or hoses connected to the tank, in which case it may be preferable to replace the load cells with pressure cells which function as described above.
If desired, the installation may be in the form of a portable apparatus. Thus, the reservoir and the associated slurry withdrawal means, volume control means and separate gravity control means may be of a size which permits them to be taken from site to site, where they are installed, on a vehicle such as a truck or lorry, and the support may, instead of being a concrete slab, be a container, such as a portable shipping container, having an open top to permit loading thereof, for example, with particulate material by means of a front end loader, and having a closeable slot at an end thereof from which drainage of slurry can take place if the container is mounted at an angle with the slot lowermost.